1. Field of the Invention
The present invention relates to a negative electrode material for a lithium ion battery and use thereof. More specifically, the present invention relates to a negative electrode material capable of providing a lithium ion battery having a large charge/discharge capacity, and further, being excellent in the charge/discharge cycle characteristics and rapid charge/discharge characteristics, a paste containing the negative electrode material, a negative electrode sheet obtained by coating the paste, and a lithium ion battery incorporating the negative electrode sheet.
2. Description of the Related Art
Polyfunctionalization of portable electronic devices is proceeding at a high speed, outstripping the power savings of an electronic component and in turn, the power consumption of portable electronic devices is increasing. In this regard, realization of a high-capacity, small-size lithium ion battery as a main power source of a portable electronic device is more keenly in demand than ever. In addition, the growth in demand for an electric vehicle is also driving a strong need for fabrication of a high-capacity lithium ion battery used therein.
In conventional lithium ion batteries, graphite is predominantly used as a negative electrode material. Since Li which can be occluded in graphite is limited to the stoichiometric ratio in LiC6, the theoretical capacity of a lithium ion battery using graphite as the negative electrode is 372 mAh/g.
In order to realize fabrication of a high-capacity lithium ion battery, studies are being conducted for use of a particle containing a metal element having a large theoretical capacity, such as Si and Sn, as the negative electrode material. For example, when an Si-containing particle is used for the negative electrode material, the theoretical capacity of the lithium ion battery is 4,200 mAh/g. Since the theoretical capacity of a lithium battery using metallic lithium is 3,900 mAh/g, when Si or the like is used as the negative electrode material, there is an expectation that a high-capacity lithium battery of smaller size can be obtained. However, a negative electrode material such as Si exhibits a large percentage of expansion or contraction on intercalation/deintercalation (occlusion/release) of a lithium ion. As a result, a gap is produced between particles, thus failing to obtain as large a capacity as expected. Furthermore, the particles are crushed to a fine powder due to repetition of large expansions or contractions. Further, the resulting break in electrical contact leads to an increase in the internal resistance and in turn, a short charge/discharge cycle life of the lithium ion battery thus obtained.
Proposals made to solve these problems include, for example, a negative electrode material containing an Si- and/or Sn-containing particle and carbon fiber (Patent Document 1), a negative electrode material obtained by attaching a carbonaceous material containing an Si-containing particle and carbon fiber to the surface of a graphite particle (Patent Document 2), a negative electrode material composed of a mixture of a metallic particle such as Si, Sn and Ge and a graphite particle having a d002 of 0.3354 to 0.338 nm and an area ratio of G/D≧9 between the G peak and D peak as measured by Raman spectroscopy (Patent Document 3), and a negative electrode material composed of a solid solution containing an element capable of occluding/releasing a lithium ion, such as Si and Ge, and an element incapable of occluding/releasing a lithium ion, such as copper (Patent Document 4).    [Patent Document 1] JP-A-2004-178922 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    [Patent Document 2] JP-A-2004-182512    [Patent Document 3] JP-A-2004-362789    [Patent Document 4] JP-A-2002-075350